The present invention relates to a level shifter, a level shift circuit, an electro-optical device, and to an electronic apparatus which can reduce power consumption.
In recent years, electro-optical devices having an electro-optical element such as a liquid crystal or an organic light-emitting diode have been widely spread, instead of a display device having a cathode ray tube. The electro-optical devices are broadly classified as either a passive matrix type or an active matrix type. The active matrix type electro-optical device has scanning lines each extending in a row direction, data lines each extending in a column direction, and pixel electrodes provided corresponding to intersections of the scanning lines and the data lines. Furthermore, thin film transistors are provided between the data lines and the pixel electrodes at the intersections, which are turned on and off according to scanning signals supplied to the scanning lines. In addition, a counter electrode is provided corresponding to the pixel electrodes, and an electro-optical material is interposed between the pixel electrodes and the counter electrode.
In order to drive the electro-optical material or the thin film transistors, a relatively high voltage is needed. On the other hand, an external control circuit which supplies a clock signal, a control signal, and so on for driving to the electro-optical device typically has CMOS (Complementary Metal Oxide Semiconductor) circuits, and the amplitude of a logical signal is in a range of from 3 to 5 V. Accordingly, the electro-optical device generally converts a low-amplitude logical signal into a high-amplitude logical signal by using an amplitude conversion (hereinafter, simply referred to as a ‘level shifter’).
As the level shifter, a level shifter in which an input signal is input via an alternating current (AC) coupling is known (for example, see Patent Document 1).
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2003-110420 (claim 1 and Abstract)
The level shifter converts the signal level, but it is not necessary to operate continuously according to an action of the signal to be converted. For example, in an electro-optical device for color display, there is a case in which a demultiplexer is used to output multiplexed data line signals corresponding to respective colors of R, G, and B from the data line driving circuit via one output terminal and distribute the multiplexed data line signals to data lines for R, G, and B. The demultiplexer outputs the data line signals to the data lines of the respective colors according to color selection signals of R, G, and B which become exclusively active. In this case, it is common practice to convert the signal levels of the color selection signals by means of the level shifter. As such, the level shifter is not needed to continuously operate, and it may operate only during a period in which the input color selection signal becomes active.
On the other hand, in a conventional level shifter, when a power is supplied, some of the transistors are not completely turned off, so that a small current flows between a source and a drain of the transistor. For this reason, there is a problem in that power is consumed during a period in which it is not necessary to operate originally.
Accordingly, the present invention has been made in consideration of the above-mentioned problems, and it is an object of the present invention to provide a level shifter, a level shift circuit, an electro-optical device, and an electronic apparatus having reduced power consumption.